Scioly Rocks and Minerals

https://scioly.org/wiki/Rocks_and_Minerals

Igneous Rocks

Two categories: intrusive and extrusive

Intrusive rocks

Type of ingeous rock formed from magma that cools slowly beneath the Earth's surface. Often forms large mineral crystals within the rock. Ex: granite

Extrusive rocks

Type of igneous rock that cools quickly during a volcanic eruption and is usually fine-grained or glassy, such as obsidian and basalt. May have large crystals called phenocrysts, but the rest of the mass is fine-grained

Phaneritic rocks

are the most common igneous rocks, and are rocks with large crystals, formed by crystallizing slowly inside the Earth. They are intrusive. As it cools, the minerals have time to form crystals large enough to see with the naked eye. Phaneritic rocks are called phanerites.

Aphanitic rocks

are those with very fine grains, so the individual crystals are unable to be seen with the naked eye. They form from lava which crystallizes quickly near/on Earth’s surface, and therefore are extrusive. When they make contact with the atmosphere or somewhere close to the surface they cool rapidly, and do not have time to form large crystals.

Porphyritic rocks

are those that display crystals with differences in size. Normally they are bimodal, meaning the crystals within are of two distinct sizes. The large crystals are called phenocrysts. In extrusive rocks the phenocrysts are normally in an aphanitic matrix; in intrusive rocks the phenocrysts have a matrix with crystals visible with the naked eye (aphanitic) and phenocrysts that are notably larger. They form when there is a change in temperate during cooling, meaning both intrusive and extrusive rocks can be porphyritic.

Glassy/vitreous rocks

form when the magma cools too fast for crystallization to happen, leaving behind a smooth natural glass with few/no crystals. A very prominent example is obsidian.

Sedimentary rocks

occur when smaller pieces of rock and sand are cemented together. They have two types: clastic or organic

Clastic rocks

Form from other rocks and minerals. Ex: sandstone

Organic rocks

Form from the bodies or shells of other organisms. Ex: limestone and coal

Metamorphic rocks

rocks that are composed of other rocks that have been subjected to heat and pressure. They often bear little resemblance to their parent rocks (protoliths)

Contact metamorphism

is caused by the temperature increase when magma enters a rock. The area affected is called the aureole. Rocks made from this are usually called hornfels, and do not show strong signs of metamorphism. They are often fine grained and very tough.

Dynamic metamorphism

is caused by rocks grinding against each other, known as mechanical deformation. This leads to pulverization and the breakage of rocks. New minerals can also form when this happens. This is most commonly seen at fault zones, where tectonic plates are rubbing against each other.

Regional metamorphism

is when a large portion of the Earth’s crust undergoes metamorphosis. A lot of times this refers to dynamothermal metamorphism, which happens due to high temperatures and pressures at convergent plate boundaries. This is when two tectonic plates smack into each other.

Minerals

Have definite chemical and crystal structures

Groups

A way to classify minerals based on chemical makeup 

Special phenomena

Magnetism, fluorescence, or triboluminescence

Formula

The specific chemical composition of the mineral

Streak

The color left when the rock or mineral is rubbed against an unglazed piece of porcelain. It is more accurate than color for identifying minerals

Luster

The way a mineral reflects light

Common types of luster

Vitreous (glassy) adamantine (brilliant or gem-like) resinous (resin-like) greasy, pearly, waxy, and silky

Crystal structure and crystal habit

The basic overall shape of the mineral as it grows

Cleavage

The tendency of a mineral to break a long smooth, flat surfaces. It can be described as perfect, good, distinct, or poor.

Fracture

The way the mineral breaks (not along a cleavage plane)

Specific Gravity

The measure of how dense a mineral is in comparison to water